He's right, Night Creatures are essentially a special semi-megabeast that lives in small caves that aren't connected to the Cavern network. You can find Night Creature lairs pretty easily if you spot the entrance; unlike most caves there is a hatch/door covering the entrance. Inside you will find a Mortar & Pestle for grinding bones, a cauldron and maybe some other things. They typically only wield a 'carving knife'. Amusingly my first Night Creature kill was a 10-foot tall three-eyed Ogre that I brained in one mace strike.
Bogeymen are much different; they are not Night Creatures because they don't show up in world gen at all, they won't attack people unless they travel alone, etc. This doesn't make them any less dangerous however as hundreds of them can show up and chase you around the map until you either run far enough from the place you found them, kill every last one, or reach a site (cave/old fortress/town/etc.). They appear to be generated with characteristics similar to Night Creatures, if you look at them individually they are all built with different parts and looks. But when defeated or left behind by running, they vanish into smoke. This makes it nearly impossible to procure Bogeymen body parts. Just save yourself some trouble and bring along a drunk, a slave child or elite soldier or whatever.